Process of preparing polyethylene terephthalate using metal thiocyanate transesterification catalysts



United States Patent 3 454,531 PROCESS OF PREiARING POLYETHYLENE TEREPHTHALATE USING METAL THIOCY- ANATE TRANSESTERIFICATION CATALYSTS Mary J. Stewart, Media, and John A. Price, Swarthmore, Pa., assignors to FMC Corporation, Philadelphia, Pa., a corporation of Delaware No Drawing. Filed Nov. 23, 1966, Ser. No. 596,459 Int. Cl. C08g 17/013, 17/015 U.S. Cl. 26075 6 Claims ABSTRACT OF THE DISCLOSURE Process of preparing polyethylene terephthalate comprising carrying out a transesterification reaction between dimethyl terephthalate and ethylene glycol in the presence of a metal salt of thiocyanic acid and then polycondensing the resulting product.

This invention relates to a method of preparing filament-forming linear polyesters. More particularly, it relates to an improved method for preparing filament-forming polyethylene terephthalate through the use of a transesterification catalyst.

The manufacture of filament-forming polyester resin from a dialkyl terephthalate and glycol is well-known in the art. Generally, in the preparation of such polyesters, a dialkyl terephthalate and glycol are first combined and subjected to an ester-interchange reaction or transesterification in the presence of an ester-interchange catalyst at elevated temperature and atmospheric pressure. The resulting product or prepolymer is then polycondensed at higher temperatures and under reduced pressure in the presence of a polycondensation catalyst to form the desired filament-forming polyester resin.

In general, a polyethylene terephthalate resin suitable for melt spinning into filaments should have a carboxyl content value of about or below 50 equivalents per million grams (eq./ gr. or meq./kg.). Additionally, a polyethylene terephthalate suitable for such use should have a melting point of at least about 258-260 C. and an intrinsic viscosity of not less than about 0.60, as determined in a 60% phenol and 40% tetrachloroethane solution (wt/wt.) at 30 C.

From a commercial standpoint, it is essential that the polyester resin be producedin the shortest possible time and the desired degree of polymerization be obtained. One way of evaluating the effectiveness of a transesterification catalyst is by measuring the .half-time of the catalyst. The half-time is defined as the time necessary for the first one-half of the theoretical amount of methyl alcohol that will be produced during the transesterification reaction to distill from the transesterification mixture. It is desirable that the half-time be as short as possible; however, it is essential that the transesterification catalyst in any cast acts to produce a polyester prepolymer suitable for polycondensation to a high molecular weight polyester.

It is an object of this invention to provide a method for preparing polyethylene terephthalate resin.

Another object of the present invention is to prepare polyethylene terephthalate resin suitable for spinning into processable filaments.

An additional object of the present invention is to provide an improved method for accelerating the transesterification method between ethylene glycol and dimethyl terephthalate in the preparation of polyethylene terephthalate.

These and other objects are accomplished in accordance 3,454,531 Patented July 8, 1969 ice with the present invention which involves a method of preparing polyethylene terephthalate wherein dimethyl terephthalate and ethylene glycol are transesterified and the resulting transesterified product is polycondensed in the presence of a polycondensation catalyst, the improvement comprising carrying out the said transesterification reaction in the presence of a catalytic amount of a metal salt of thiocyanic acid.

The metal salts of thiocyanic acid that are used as catalysts in the transesterification step of the present method may be varied to meet any requirements of reaction conditions and desired product. While the present invention is not to be limited to any particular suitable metal salts of thiocyanic acid, it has been found that the preferred salts are those formed with the metals from Groups II-A, II-B, IV-A, and VIII of the Periodic Table (see Merck Index, sixth edition, inside front cover). For example, among the transesterification catalysts that can be used in the present method are zinc thiocyanate, lead thiocyanate, calcium thiocyanate, and cobalt thiocyanate, or any combination thereof.

Generally, concentrations of the present transesterification catalyst in the range of from about 0.01% to about 0.2%, based on the weight of dimethyl terephthalate in the subject dimethyl terephthalate-ethylene glycol reaction mixture is used. Usually, it has been found that from about 0.02% to about 0.10% of the present salts of thiocyanic acid, based on the weight of the dimethyl terephthalate in the reaction mixture, is preferred to produce linear polyester resins suitable for forming filaments. Higher or lower concentrations of the present catalyst can also be used. However, when concentrations less than the above are used, their catalytic effect is generally reduced, whereas if concentrations greater than this are used, no further improvement in the present method or desired product is obtained.

In general, the preparation of filament-forming polyesters of the present invention via the ester-interchange reaction is carried out with a molar ratio of ethylene glycol to dimethyl terephthalate of from about 1:1 to about 15:1, but preferably from about 1.2:1 to about 2.621. The ester-interchange reaction is generally carried out at atmospheric pressure in an inert atmosphere such as nitrogen, initially at a temperature range of from about C. to about 250 C., but preferably between about C. and 200 C. in the presence of an esterinterchange catalyst. During this first stage, methyl alcohol is evolved and is continuously removed :by distillation. After a reaction period of about one hour, the temperature of the reaction mixture is raised to about from 200 C. to about 300 C. for approximately 3 to 5 hours in order to complete the reaction and distill 01f excess glycol. The main and desired product of the esterinterchange reaction is the prepolymer which is comprised principally of bis(2-hydroxyethyl)terephthalate. The second stage or polycondensation step of the present method is generally achieved under reduced pressure within the range of from about 225 C. to about 325 C. for about 35 hours. It is preferable to carry out both the first and second stages of the present method under agitation.

The polycondensation step of the present method is generally accomplished through the addition of a suitable catalyst, for example, antimony trioxide, and the like. The polycondensation catalyst may be added to the present reaction mixture before initiating the ester-interchange reaction between the ethylene glycol and dimethyl terephthalate or after the product thereof is formed. The polycondensation catalysts are generally employed in amounts ranging from about 0.005 to about 0.5%, based on the total weight of reactants.

The following examples of several preferred embodiments of the present invention will further serve to illustrate the present invention. All parts are by weight, unless otherwise indicated.

EXAMPLES A mixture comprising 600 grams of dimethyl terephthalate, 96 ml. of ethylene glycol and 0.24 gram of a salt of thiocyanic acid, as listed in the following table, was charged into a reaction vessel equipped with a nitrogen inlet, a distillation arm, heating means, and stirring means. The reaction mixture was agitated and heated at atmospheric pressure to about 197 C. over a period of about 30 minutes under a nitrogen blanket whereby byproduct methyl alcohol was distilled Off. The reaction mixture was held at about 197 C. for about 2 hours. Then, the temperature of the reaction mixture was allowed to rise in order to distill off any remaining byproduct comprising methyl alcohol and excess ethylene glycol, and form the desired polyester prepolymer product. After about 30 minutes, the temperature had risen to about 230 C. at which time the formed prepolymer was cooled under a nitrogen blanket.

'50 grams of the above prepolymer was combined with 0.02 gram of antimony trioxide in a reaction vessel equipped with a nitrogen inlet, a vacuum source, a distillation arm, stirring means, and heating means. The pressure within the reaction vessel was reduced to from about 0.1 to about 0.05 mm. of mercury at 285 C. under a nitrogen blanket and the reaction mixture was agitated under these conditions for about three hours to bring about the polycondensation of the prepolymer and formation of the polyester resin.

The following table sets forth the results of various formed which is suitable for polycondensation into a highly polymeric polyester resin. The resulting polyester products have high molecular weights, as indicated by their intrinsic viscosities, high melting points, and sufficiently low carboxyl content values so as to make them suitable for use in preparing polyester filaments.

It will be apparent that various different embodiments of this invention may be made practicing this invention Without departing from the spirit and scope thereof and therefore, it is not intended to be limited except as indicated in the appended claims.

We claim:

1. In a process for the preparation of polyethylene terephthalate resin wherein dimethyl terephthalate and ethylene glycol are transesterified and the resulting transesterified product is polycondensed in the presence of a polycondensation catalyst, the improvement comprising carrying out the said transesterification in the presence of a catalytic amount of a metal salt of thiocyanic acid transesterification catalayst wherein the metal component of said salt is from Groups II-A, II-B, IV-A, and VIII of the Periodic Table (Merck Index, sixth edition).

2. The process of claim 1 wherein the salt is present in an amount from about 0.01% to about 0.20%, based on the weight of dimethyl terephthalate in the reaction mixture.

3. The process of claim 1 wherein the salt is cobalt thiocyanate.

4. The process of claim 1 wherein the salt is calcium thiocyanate.

5. The process of claim 1 wherein the salt is lead thiocyanate.

6. The process of claim 1 wherein the salt is zinc reactions carried out as described above. thiocyanate.

TABLE Polyester Polyester, carboxyl Half-time intrinsic content, Melting Transesterification catalyst (minutes) viscosity meqjkg. Pt., 0.

Example No.:

1..-. Zinc thioeyanate (ZI1(SCN)2) 20 0. 89 36.0 264 2 Lead thiocyanate (Pb(SCN)i) 32 1.05 10.1 263 3.. Calcium thioeyanate (Ca(SCN)z) 90 0 95 17.4 259 Cobalt thiocyanate (C0(SCN)z) 33 1 0.73 55.3 263 l 4-hour polycondensation time with no stirring.

The intrinsic viscosities of the polyester products in References Cited the above examples were determined in a 60% phenol UNITED STATES PATENTS and tetrachloroethane solution (wt./wt.) at 30 C. 6

The results in the above examples indicate that the 332 ,965 6/1967 Schulthels 260-475 presence of a suitable thiocyanate during the transesterification step of the present method facilitates the preparation of and improves the polyester prepolymer formed and in turn the resulting polyester resin product. Through the use of the present method, the transesterification re action is accelerated and a transesterified product is WILLIAM H. SHORT, Primary Examiner.

LOUISE P. QUAST, Assistant Examiner.

US. Cl. X.R. 

